This invention relates generally to switching between at least two input modes and reducing transients caused by the switchover process.
Often in digital signal processing applications, it is necessary to switch between different input modes. One method of switching is called hard switching. Hard switching disconnects a first input signal before connecting a second input signal; this is colloquially called xe2x80x9cbreak-before-makexe2x80x9d switching. This method has the disadvantage of potentially causing a large transient to occur at the output of the switch, because the signal at one or more inputs to the switch may not be a zero value at the time of the switchover due to network delay. If the output of the switch is sent to a device such as a modulator in a transmitter of a cellular radiotelephone, the hard switching transient could cause the spectral emission of the cellular radiotelephone to be unacceptable to the cellular system.
Another method of switching between input modes, called soft switching, avoids the hard switching transient; however, the output of the switch may be an overflow condition which causes a soft switching transient. During soft switching, the input signals are summed at the switch, and switching is accomplished by setting all but one of the input signals to a constant zero value. Circuit delay, however, can cause more than one signal to be a non-zero value at the switchover time, and the summed value at the output of the switch might exceed acceptable levels. For example, if the output of the switch is a digital number, soft switching can cause wrapping of a value that exceeds the highest allowable digital number over to a low digital number. Wrapping can result in soft switching transients that are as bad as or worse than hard switching transients.
In cellular radiotelephone transmitter applications, when switching between input speech and data modes, most any kind of switching transient will result in unacceptable spectral emissions. In a cellular radiotelephone transmitter application, the output of the switch enters a modulator and the modulated data signal does not normally pass through a limiting or splatter filter before being transmitted. Thus, any interaction between the first input mode and the second input mode will directly affect the modulator output. There is a need for a method of switching that avoids the transients caused by hard switching and soft switching.